New Zealand Garden Bird Survey
Cartoon - Susan Marks.
New Zealand has a number of native bird species that are either resident in domestic gardens year round or visit them in winter. Most (e.g. tūī, bellbird (korimako), and kererū) are not currently of conservation concern, but it is not known whether their populations are stable, increasing or decreasing. If they are decreasing, they could be of concern in the future. Monitoring the population trends of bird species nationally is an enormous task, but one that is possible with the public’s help.
The New Zealand Garden Bird Survey, organised by Eric Spurr, is now in its sixth year. Volunteers spend one hour in midwinter observing birds in their home gardens, recording the largest number of individuals of each species seen or heard at any one time.
The two most numerous species counted to date have been the house sparrow and silvereye (Table, Fig.). However, counts vary between regions (Table). For examplhouse sparrow counts have been higher in Northland and Auckland than in Canterbury, Otago and Southland, while silvereye counts have been higher in these southern South Island regions than in the two northern North Island regions.
Counts of house sparrows increased over the first 4 years of the survey (2007 to 2010) then declined slightly in 2011 (Fig.). The survey could not determine the causes of these changes, but the increase from 2007 to 2010 may have been recovery from an outbreak of salmonellosis in 2000, which caused major mortality in house sparrow populations in many parts of the country.
Silvereye counts declined from 2007 to 2009, increased markedly in 2010, and then declined markedly in 2011 (Fig.). Again, the survey could not determine the causes of these changes, but circumstantial evidence suggests the 2008–2009 decline was at least partly caused by an outbreak of avian pox, and the 2011 decline by a mild winter prior to the count. This latter may seem counter-intuitive but the lack of snow and frosts may have meant that silvereyes in forests had no need to move into domestic gardens in search of food.
Apart from geographic region, several other factors influenced the number of birds counted in gardens, including whether the garden was urban or rural, and whether supplementary food was provided for birds. More species, and more individuals of most species, were counted in rural than in urban gardens. This was probably because rural gardens are larger and more diverse than urban gardens. However, more house sparrows, silvereyes and greenfinches were counted in urban than in rural gardens, probably as a consequence of supplementary feeding which occurred more often in urban than in rural gardens.
Gardens in which supplementary food (e.g. bread, fat, fruit, seeds, sugar-water) was providehad higher counts of some species (e.g. house sparrow, silvereye, starling, chaffinch, bellbird and greenfinch) than gardens without supplementary food. This is not surprising, because these species are attracted to the food provided. However, gardens with supplementary food had lower counts of other species, such as goldfinch, fantail and grey warbler. Goldfinch may have been supplanted at bird feeders by the bigger, more aggressive greenfinch, and the insectivorous fantail and grey warbler may have avoided gardens with supplementary food because the local invertebrate populations had been depleted by the large numbers of other species present (e.g. silvereyes).
Eric notes that the gardens surveyed are not randomly selected so the results apply only to the gardens of survey participants. Nevertheless, the survey has the potential to alert authorities to changes in garden bird populations, and to provide circumstantial evidence for the success (or otherwise) of management actions, particularly mammalian pest control. Many contributors to the survey commented on how the number of birds in their gardens had increased since local authorities had undertaken pest control in nearby forest reserves.
This year’s survey is planned for 30 June – 8 July. Anybody able to identify birds in their garden can take part. Details will be placed on the Garden Bird Survey website close to the time of the survey.
This work was supported by Forest and Bird, New Zealand Ornithological Society, and Topflite bird foods.
Eric Spur
Table. Average maximum number of birds counted at any one time per garden for 22 species in the four major regions in 2011. The national average was calculated from all regional averages weighted by the proportion of all households in each region.
Fig. Average counts of the 12 species that have occurred in the top 10 in one or more years, 2007–2011 (calculated from regional average counts weighted by regional proportions of all households).
